Recently, liquid crystal display devices represented by monitors (displays) for personal computers and television receivers are in widespread use as various display means. In these liquid crystal display devices, it is known that visibility of display is enhanced by technology such that a polarizer is arranged at each side of a liquid crystal cell and further a retardation film is provided between the liquid crystal cell and the polarizer.
Herein, a retardation film is formed in a rectangle shape to fit on a liquid crystal display device. Such a retardation film for use in the liquid crystal display device may be required to be molecularly oriented in an oblique direction relative to each side of the rectangle shape. This kind of film is generally produced by stretching or relaxation heat treating of a strip-shaped polymer film so as to be molecularly oriented in a desired direction. Such methods for producing a stretched film include a method disclosed in Patent Document 1 specified below.
In the invention disclosed in Patent Document 1, when a film (polymer film) is conveyed in a longitudinal direction while both side edges of the film are held, a distance between a starting point and a releasing point of holding of one side edge is made longer than a distance between a starting point and a releasing point of holding of the other side edge. By this arrangement, though holding positions of the both side edges are located at the same position in the longitudinal direction (conveying direction) at the beginning, a holding position of the one side edge lies behind a holding position of the other side edge in the conveying direction. Therefore, the holding position of the other side edge is pulled backward in the conveying direction, so that the film is stretched in an oblique direction.
Stretching or relaxation heat treatment of a polymer film may disadvantageously wrinkle the polymer film. Wrinkles of the polymer film may damage flatness of a retardation film to be formed, resulting in deterioration of quality. Hence, various methods have designated to stretch or carry out relaxation heat treatment on a polymer film with preventing or removing wrinkles formed on the polymer film. Such methods include, for example, a method disclosed in Patent Document 2 specified below.
The method disclosed in Patent Document 2 suppresses wrinkles of a stretched film by giving a tensile force on the film in a width direction and simultaneously providing a means to support the own weight of the film when the stretched film undergoes relaxation heat treatment.
There are the other known arts described in Patent Documents 3 and 4 as methods for producing a stretched film.
The invention disclosed in Patent Document 3 outwardly broadens both side edges of a stretched film when the stretched film is conveyed in a longitudinal direction with its both side edges held. By the invention disclosed in Patent Document 3, the both side edges of the stretched film are further moved inwardly.
The invention disclosed in Patent Document 4 curves overall a conveying path of a stretched film when the stretched film is conveyed in a longitudinal direction with its both side edges held.